It is a common practice in some industrial ovens or furnaces to provide fuel-fired radiant-heating tubes which extend into the furnace and have, at an end external of the furnace, an outlet for the combustion exhaust gas. Such tubes are supplied with fuel and combustion-sustaining gas, e.g. air, and are heated by a combustion process within the tube.
Customarily, the fuel is delivered by a pipe extending axially through the heating tube while the combustion-sustaining air is delivered by a fitting at the aforementioned end of the heating tube to a space surrounding the fuel-supply pipe and defined by a further duct, tube or sleeve which defines another annular passage outwardly of this duct, i.e. between the duct and the wall of the heating tube, through which the exhaust gas of the combustion process is led in the opposite direction to the aforementioned outlet.
In other words, the fuel and combustion-sustaining gas are led through inner coaxial paths through the tube from the end thereof which extends out of the furnace wall, sustain combustion within the tube to produce hot exhaust gases, and the latter exhaust gases are diverted back again in an outer passage toward the end of the tube projecting through the furnace wall to be discharged from an outlet outside the furnace. The combustion air is in heat-exchanging relationship with the exhaust gases along the aforementioned duct which can be a thin-wall metal member or some other body of heat-conducting material.
As a consequence, the heating tube is provided with a recuperative heater whereby a portion of the exhaust gas heated is transferred to the combustion-sustaining gas within the heating tube.
The internal recuperative preheating effect is seldom sufficient to permit effective recovery of available heat of the exhaust gas, especially when the recuperative process is carried out over a length of the heating tube corresponding to the wall thickness of the furnace and such wall thickness is limited. In addition the exhaust gas emerging from the aforementioned end of the heating tube is a noise generator of considerable significance, the operation of this system being therefore detrimental to the environment and deleterious to operating personnel.